Hungarian emergency workers shovel plaster into the river Marcal from a bridge in Morichida, 149 kms west of Budapest, in order to prevent poisonous chemical sludge from reaching the rivers Raba and Danube. Photograph: Karoly Gyoeri/EPA

Hungary opened a criminal inquiry yesterday into the toxic sludge spill that killed at least four people after a reservoir burst at an aluminium plant.

As workers struggled to deal with the flood, the EU urged authorities to do everything they can to keep the slurry from reaching the Danube and affecting half a dozen other countries. Greenpeace yesterday described the spill as "one of the top three environmental disasters in Europe in the last 20 or 30 years".

Last night workers were pouring 1,000 tonnes of plaster into the water to try to bind the sludge and keep it from flowing into the Danube, just 45 miles away. On Tuesday, experts said it would take the sludge about five days to reach the river, one of Europe's key waterways. South of Hungary, it flows through Croatia, Serbia, Bulgaria, Ukraine, Moldova and Romania before emptying into the Black Sea.

The Hungarian prime minister, Viktor Orbán, said authorities were caught off-guard by the disaster because the reservoir had been inspected only two weeks earlier and no irregularities had been found. A criminal investigation has been opened, said police spokeswoman Monika Benyi.

Hundreds of people had to be evacuated after the gigantic sludge reservoir burst on Monday at a metals plant in Ajka, a town 100 miles southwest of Budapest, the capital. At least four people were killed, three are still missing and 120 were injured as the torrent flooded homes, swept away cars and disgorged 1m cubic metres (35m cubic feet) of toxic waste on to several nearby towns.

The reservoir was no longer leaking yesterday, but a protective wall was being built around its damaged area.

The EU said it feared the toxic flood could turn into an ecological disaster for several countries and urged Hungarian authorities to focus all efforts on keeping the sludge away from the Danube.

"It is important that we do … everything possible that it would not go, that it would not endanger the Danube," said the EU environment commissioner, Janez Potocnik. "We have to do at this very moment everything possible … [to] limit the extent of the damage."